During continuous casting of aluminum, the solidification and formation of the strip occur in contact with forged caster shells having an outer diameter of about 0.5-1.0 meters (21-40 inches). The casting speed is nominally about 1 RPM. During operation the caster shells are water cooled. The maximum temperature is approximately 575.degree. C. to about 650.degree. C., while the minimum temperature is as low as about 40.degree. C. Thus, the surface of the shell is exposed to a temperature variation of about 600.degree. C. with a cycle time of about 1 minute. The rapid thermal cycling has the consequence that extensive thermal fatigue cracking or "heat checking", takes place. The resulting crack pattern of the caster shell is imparted to the aluminum strip, thus producing unacceptable aluminum product. In operation, the caster shell must be disassembled and the crack pattern removed by machining. Extensive or repeated heat checking reduces the service life of the caster shell and increases the down time and the operating cost of the casting apparatus.
U.S. Pat. No. 4,409,027 issued to Cordea, et al. on Oct. 11, 1983 discloses a steel for use in caster shells for continuous aluminum casting. The steel described therein has a carbon content of about 0.53% to 0.58% and a chromium content of about 1.5% to 3.0%. While this steel is commonly currently used for caster shells, it is still prone to heat checking.
Literature references to thermal fatigue, thermal cracking, high temperature alloys and alloying elements include:
Nes, E. and Fartum, P., "Thermal Fatigue of Caster Shell Steels", Scandinavian Journal of Metallurgy, 1983, Vol. 12, pp. 107-111.
Sandstrom, R., Samuelsson, A., Larsson, L., and Lundberg, L., "Crack Initiation and Growth during Thermal Fatigue of Aluminum Caster Shells", Scandinavian Journal of Metallurgy, 1983, Vol. 12, pp. 99-105.
Chavanne-Ketin, "Shells and Roll Cores for Aluminium Continuous Casters", pp. 1-19.